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The ASCPC makes sure the cabin altitude gets to the target altitude when the airplane gets to cruise altitude. Scheduled step climbs are included in the calculation of the cabin rate of climb. Unscheduled step climbs cause the ASCPC to hold the cabin altitude at the value it is at when the airplane stops its climb. When the airplane starts to climb, the ASCPC returns to the calculated rate of climb for the cabin.
The ASCPC holds the cabin altitude at the target value when the airplane is in the cruise mode. When the airplane descends more than 1000 feet (305 meters), the ASCPC sets the target cabin altitude to 0.1593 psi more than the landing field pressure. The cabin altitude descends at a rate that lets the cabin get to its target altitude at the same time the airplane lands. The ASCPC makes sure the differential pressure limit does not go above the
8.6 psi limit.
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AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILES

La Paz (Bolivia) to Miami (USA)
This shows a flight profile that starts at an airport above the 8,000 feet (2440 meters) cabin altitude limit. The ASCPC sets the target for the cabin altitude the same as it did in the example for the flight from Seattle to Denver.
Addis Ababa (Ethiopia) to Bombay (India)
This shows a flight profile that starts near the 8,000 feet (2440 meters) cabin altitude limit and ends near sea level. The ASCPC sets the target for the cabin altitude the same as it did in the example for the flight from Seattle to Denver.
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AIR CONDITIONING -CABIN PRESSURE CONTROL SYSTEM -FLIGHT PROFILES

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777 AIRCRAFT MAINTENANCE MANUAL

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AIR CONDITIONING -CABIN PRESSURE RELIEF SYSTEM -INTRODUCTION

Purpose
The cabin pressure relief system limits the internal pressure of the airplane.
General Description
Two positive pressure relief valves bleed air overboard if the internal pressure increases above a safe limit. This limit is above the normal pressure in the airplane.
Four negative pressure relief vents bleed air into the airplane if ambient pressure is higher than the pressure in the airplane. This can occur during rapid airplane descent.
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AIR CONDITIONING -CABIN PRESSURE RELIEF SYSTEM -INTRODUCTION

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AIR CONDITIONING -CABIN PRESSURE RELIEF SYSTEM -COMPONENT LOCATIONS

Component Locations
The positive pressure relief valves are adjacent to each other and behind the left sidewall lining of the forward cargo compartment. The valves are opposite the forward cargo door at station 603.
Two negative pressure relief vents are on each side of the airplane. These vents are at stations 815 and 857, behind the sidewall lining of the forward cargo compartment.
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AIR CONDITIONING -CABIN PRESSURE RELIEF SYSTEM -COMPONENT LOCATIONS

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AIR CONDITIONING -CABIN PRESSURE RELIEF SYSTEM -POSITIVE PRESS RELIEF VALVE AND FILTER

Purpose
The positive pressure relief valve opens when cabin air pressure is higher than ambient pressure by a set value. This permits air to go out of the airplane to keep differential pressure below a safe limit.
Physical Description
The positive pressure relief valve is a mechanical device. It has an air filter and two ambient pressure sense connections. The air filter supplies cabin pressure to the valve. The ambient pressure connections supply ambient pressure to the valve.
An integral ambient sense port (not shown) is in the base of the valve. A remote ambient sense port is in the side of fuselage aft of the pressure relief valves. Tubing connects the sense ports to the valve.
Functional Description
When cabin pressure differential at the remote ambient pressure sense port increases to 8.95 psi, the relief valve opens. This permits air to go out of the airplane. If the remote pressure sense port does not function, the relief valve opens when cabin pressure differential at the integral ambient sense port increases to 9.42 psi.
When cabin pressure differential goes below the limit, the positive pressure relief valve closes.
If the positive pressure relief valve opens, small doors open on the outer surface of the valve. This shows a brightly colored edge on the valve doors. These doors stay open when the valve closes. The maintenance technician must close these doors manually.
　
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